Approximately how much of an organism's genetic material is contributed by its mother?

About 50%

Both the mother and father contribute about the same amount of genetic material to their offspring, but it's not quite 50/50. Taking humans as an example, both parents contribute the same amount of genetic material for 22 of the 23 chromosome pairs, but the 23rd pair that determines gender (the X and Y chromosomes) are of different size. So for female children, both parents contribute about the same amount, but for male children, the mother contributes a larger portion since the X chromosome is quite a bit larger than the Y chromosome from the father. Another area in which the contribution differs is the mitochondrial DNA which is contributed solely by the mother. For mammals, this is about 1% of the total genetic material.